When I left-click on the bottom or top scrollbar arrow, the page advances really quickly, and then stops; IOW, it advances MORE than the amount, I'm clicking. There seems to be no rhyme or reason; sometimes I can click 3,4,5X, and the page only moves the amount I click, and then the next time I click the page scrolls-ahead all by itself. Then, sometimes, I'll only click the arrow 2X, then it scrolls all by itself. Then, the next time I might click 7X (always with a pause in between clicks - it's not like I'm holding it down), and then the 8th time I click, it'll go racing down the page. Alternately, when I use the scrollbar itself, it doesn't advance as quickly as I'm sliding it; IOW, when I hold-down the left mouse button with my left hand, and move the scrollbar with my right, the scrollbar doesn't move, when I move, then all of a sudden it catches up (ie, the scrollbar might be at the top, and I've advanced the scrollbar to mid-page, but the page hasn't moved; then, all of a sudden the scrollbar catches-up and advances the page to the mid point). Firefox 50.0 / Windows 7